Bibliografia
- Rogier Noldus. CAMEL: Intelligent Networks for the GSM, GPRS and UMTS Network
- 3GPP TS 22.30 Man-Machine Interface (MMI) of the User Equipment (UE)
- 3GPP TS 22.90 Unstructured Supplementary Service Data (USSD) - Stage 1
- 3GPP TS 23.90 Unstructured Supplementary Service Data (USSD) - Stage 2